The present invention relates to a pressure cylinder comprising a piston axially displaceable within a cylinder housing and able to be pressurized by a pressure medium, a piston rod extending through at least one cylinder cap closing off said cylinder housing at one end, wherein a switching disk is provided to be displaceable on, and in coaxial frictional engagement with, the piston rod interacting with a switch to determine the position of the piston on the inside.
Such a pressure cylinder is preferably used in the textile machine industry. In a drafting arrangement for fiber material, the pressure cylinder is used as a loading device. A drafting arrangement essentially comprises an arrangement of top and bottom rollers, and a plurality of pressure cylinders applying a load to the top rollers via pressure rods. Accordingly, the support of the top roller is usually of the floating type. Fiber material is continuously passed between the rollers and drafted, i.e. extended, by a series arrangement of a plurality of such drafting arrangements. To do this, the individual drafting roller arrangements are operated at different speeds.
European Patent Application EP 0 939 151 A2 discloses such a drafting arrangement where the loading device is equipped with pressure cylinders. The loading device essentially comprises a loading arm for the top rollers of the drafting arrangement. The three top rollers are adjacent to associated bottom rollers. The top rollers are rotatably supported at both ends in corresponding bearings and are pressed against the bottom roller by the loading device in order to create a permanent frictional engagement with the passing fiber material, so the latter can be extended when it passes through the drafting frame. The spaces between the top rollers are made to be adjustable because different spaces are required depending on the fiber material to be stretched. The pressure force on the top rollers generated by the loading device is controlled by a pressure of a pressure medium in the form of pressurized air applied to the loading device via suitable pressure lines. The loading arm is also pivotable about a pivot point, so that the loading arm may be opened to make the drafting arrangement accessible when new fiber material is to be inserted or malfunctions are to be removed.
The loading devices used for this purpose are essentially pressure cylinders in which one piston is arranged to be unilaterally pressure-loaded. Pressurized air is supplied to the side of the piston facing away from the piston rod so that the pressure force generated is transferable to the top roller via the piston rod. A switching-off device is integrated into the loading device. The switching-off device serves to control the textile machine and provides electrical signals to the textile machine's control unit. It serves to detect the position of the piston rod in order to determine the load on the top rollers of the drafting frame so that the drafting frame may, for example, be switched off in case of malfunction.
In the operation of such drafting frames it may occur that so-called muffs are formed on the top rollers, i.e. that fiber material gets wound on the top roller. This leads to a critical operational state, since it may result in the bearings of the top and bottom rollers being excessively loaded, and the covers of the top rollers which create frictional engagement may be damaged. Muffs of this type adhere extremely well to the top rollers and are usually only detached with great difficulty. The switching-off device thus generates a signal evaluated by the control unit and shutting down the drafting frame if necessary. The switching-off device essentially comprises a switch that can assume two switching positions. In one of the switching positions the switch contacts a contact disk while in the other of the two switching positions, the switch is adjacent to the pressure cylinder or between the switching disk and the pressure cylinder. The switch in the present example is formed as an electrically conductive disk in a sliding fit on the piston rod, resulting in a frictional engagement with the piston rod. When the piston rod moves, the switching disk is displaced accordingly along the piston rod. The switching-off device is positioned in the cylinder cap.
Now, if a muff is formed, the floating top roller is pressed towards the pressure cylinder. The pressure rod of the pressure cylinder contacting the top roller is thus moved toward the retracted end position. When retracting the pressure rod, the switching disk mounted in frictional engagement passes through the switch, thus switching off the drafting frame. The drafting frame may now be manually opened and the muff removed. Afterwards, the drafting frame can be brought back into its operating position by closing the loading arm.
In practice it has been found that a readjustment of the pressure rod of the pressure cylinder may occur when such work is carried out. If the pressure rod is pushed back, this is an uncritical condition since on pressurizing the pressure cylinder again, the switching disk is carried along into its bottom end position by the piston rod. If, however, the pressure rod is extended, the pressure rod will be pushed back again when the loading arm is closed. Due to the frictional engagement on the pressure rod, however, the switching disk will be pushed into its top end position. This end position will then erroneously be recognized as a muff by the switching-off device, which is interpreted by the control unit accordingly, preventing the drafting frame from resuming its operation. The problem with the above prior art is therefore that the prior switching-off device may report undefined operational states.